"When Worlds Collide, Part 1" is the fifth episode of the fifth and final season of Teenage Mutant Ninja Turtles (2012 TV series), the one-hundred and ninth episode overall in the series, and is also part 1 of 2 of the When Worlds Collide dual-episode event.

Production-wise, it is the tenth episode of Season 5, the one-hundred and thirteenth episode overall, and is also part 1 of 2 of the mid-season finale of Season 5. It first aired on June 18th, 2017.

Plot

Somewhere in outer space, a Salamandrian fighter force travels through the cosmos, lead by Mona Lisa and her superior, Sal Commander. However, their general R'Kavaka soon comes on a hologram with news that a rogue Salamandrian named 'K'Vathrak' (whom Sal Commander has a grudge against as it was that rogue Salamandrian who cost him his left eye) had been spotted on Earth and is hunting benevolent Utrom. Realizing that planet Earth is where Raphael lives, Mona and Sal Commander break off from the fleet to head to Earth.

In New York City, three Utrom hunt for the rogue, who ambushes them in an alleyway. It is implied that they are killed by the Salamandrian.

Meanwhile, after watching the latest episode of "Space Heroes: The Next Generation," Raphael is ticked off by how each of his brothers has a love interest and he doesn't. April and Donnie are too busy having a popcorn fight to watch Space Heroes; Leo and Karai are playfully sparring in the dojo whilst exchanging some flirtatious banter, during which Karai states that even though she has a crush on him, she won't go easy on him (a possible reference to tough love) while Mikey appears to be talking to Renet, having formed a long distance relationship with her.

Raph storms out to his room to vent his anger on his drum set, in addition to sulk and rant his complaints to Chompy, only to soon get a call over his space helmet, that Fugitoid gave him, from Mona, who reports that she is on her way to Earth with Sal Commander. Raph is ecstatic by this. When Mona and Sal Commander land on a rooftop and meet up with April, the Turtles, and Karai, we see Raphael immediately embrace Mona Lisa. Seeing this, Karai finds it weird that Raph is in love with a newt, but also shows some unsettled discomfort around Leo's comments on Raph having weird taste, which is basically reminding her that she herself is dating a Turtle. After this, Mona Lisa and Sal Commander explain why they are there, and the Turtles realize the rogue Salamandrian is really the Newtralizer, going after Utroms and travelling back and forth between Earth and Dimension X to heedlessly wreck havoc without mercy.

Meanwhile, at the Utrom High Council, Queen, Pawn, and Rook are concerned about what's happening, and ask Bishop to investigate. Bishop agrees to take an Earth Protection Force commando team to deal with the Newtralizer and the recent arrivals of Mona and Sal Commander on Earth as well. Soon enough, Bishop and the EPF commandos attack Sal Commander and Mikey after they split up from Raphael and Mona.

Before going after them, Mona explains to Raphael that she and Sal Commander had to deal with Lord Dregg, who survived his presumed death by the Triceratons, and managed to still launch his invasion of Salamandria. They fought him off, but Dregg swore to return.

After rescuing Mikey and Sal Commander, they are surrounded by Bishop and the EPF commando team. Thankfully, once Raphael and Mikey explain that Mona and Sal Commander are the ones who helped the Utroms previously in Dimension X with the Kraang, Bishop and the EPF release them and agree to aid them against Newtralizer, with Bishop acting as the bait to draw Newtralizer out. Karai and Leonardo team with Donatello and April to investigate and ambush the Newtralizer. Despite their best attempts, Newtralizer proves too powerful, and once Leo and Karai (along with Donnie and April) arrive before Newtralizer can eat Bishop, who has them retreat back to the Utrom High Council chamber, where Rook reveals a device that could help deal with Newtralizer, but he arrives, disables Queen's Miss Campbell body, decapitates Rook, and nearly eats Pawn, before Bishop leads the Turtles and everyone else back to NYC. However, as they fight Newtralizer at a power substation where he fought Leo, Karai, Donnie, and April earlier, where April weakened him with water, he manages to apparently electrocute and disintegrate Mikey. Enraged, Raphael zaps him with Rook's weapon, but before it can destroy him completely, electromagnetic interference saves Newtralizer.

The reason soon presents itself: Newtralizer's old friend and ally, and the one that the Turtles hoped they would never see again: Lord Vringath Dregg, now in a new living command ship, which quickly begins laying Vreen eggs to begin taking over the city. As Newtralizer retreats to the ship, the Turtles, April, Karai, Salamandrians, and Bishop look up in disbelief and shock as things have just gone from bad to worse. The episode concludes with Karai and Leonardo looking up into the sky in horror as Lord Dregg's ship hovers overhead, and Vreen insects start infesting the city.

Trivia

When April uses water to short out Newtralizer's tech, she creates the illusion that she's waterbending, similar to Katara from Avatar: The Last Airbender, who Mae Whitman also provided the voice of.

The captions spell Renet's name as Grenette.

This episode reveals the following:

G'Throkka has a bionic eye because Newtrailzer took out his real one and left him to perish by his wound.

Renet is seen without her helmet on. Yet when she took her helmet off in The Crypt of Dracula, Michelangelo looks at her as if it were the first time he saw her without it. This was due to the episodes being aired out of production order as this episode was actually meant to be aired after the "Monsters vs Mutants" saga